If I do this: 

    def onchange_planned(self, cr, uid, ids, planned=0.0, effective=0.0):

        if self.pool.get('res.users').has_group(cr, uid, 'project.group_project_manager'): # return True if group is assigned to user, False otherwise

            #your logic goes here if user exists in the group

            return {'value': {'remaining_hours': planned - effective}}

        raise osv.except_osv(('Info'), ('You are not available to change the planned time settings.'))

Other user get the raise info message but can change the planned_hours. Why? How Can I stop that a user not in this group can't CHANGE the planned hours???????